Since ethylene/α-olefin rubbers represented by ethylene/propylene/non-conjugated diene copolymer rubber (EPDM) have no unsaturated bonds in the backbone of their molecular structure, they exhibits thermal resistance and weatherability superior to those of generally used conjugation diene rubbers, and accordingly, they are widely used in automobile parts, materials for electrical wire, materials for construction and civil engineering, parts of industrial members, modifiers for various resins, and so on.
When ethylene/α-olefin system rubbers are crosslinked by using peroxide, and in particular, when they contain an non-conjugated polyene such as 5-vinyl-2-norbornene (hereinafter, also referred to as VNB) as a copolymerization component, they exhibit a rapid crosslinking rates.
However, ethylene/α-olefin/VNB terpolymers produced by using existing catalysts have many long-chain branches derived from terminal vinyl groups of VNB. In this case, since many terminal vinyl groups of VNB in the copolymers have been consumed, improvement in crosslinking rate is not sufficient, and moreover, the long-chain branches may worsen the processability of the copolymers during their formation and their physical properties after the formation. Such long-chain branches may also be generated by using vanadium-based catalysts, and the long-chain branch content was observed to have a tendency to increase especially when metallocene-based catalysts are used for polymerization.
Patent document 1 and Patent document 2 describe ethylene copolymers which are polymerized by using metallocene-based catalysts and contain structural units derived from ethylene, an α-olefin, and VNB, and Patent document 1 describes that the copolymer is suitable for foam shaping, and Patent document 2 describes that a rubber shaped article can be formed which has an excellent surface appearance, strength property, heat aging resistance, and light resistance, and a small compression set. However, ethylene copolymers obtained by these techniques have a large long-chain branch content.
Patent document 3 describes a process for producing a polymer containing monomer units of ethylene, an α-olefin, VNB, and 5-ethylidene-2-norbornene (ENB) by using a Group 4 metal compound having a single cyclopentadienyl ligand and a monosubstituted nitrogen ligand, aluminoxane and a catalyst-activating agent as a catalyst system, and also describes the production of an EPDM polymer having a high VNB content and a low degree of branching. However, the EPDM polymer described in Patent document 3 has a problem that it is unsuitable to obtain a shaped article having an enough heat aging resistance because of too many dienes per molecule of the copolymer.
In this situation, a novel ethylene/α-olefin-based rubber has been required which contains, with a small long-chain branch content, a non-conjugated polyene such as VNB as a copolymerization component.
Styrene-butadiene rubber (SBR) is conventionally widely used for tires, for example, for automobile uses. Diene rubbers such as styrene-butadiene rubber singly have insufficient weatherability, and when they are used for a long outdoor use such as for a tire use, an amine-based rubber antioxidant or a paraffin-based wax are usually added to improve weatherability. However, diene-rubber products in which the amine-based antioxidant or the paraffin-based wax are blended may undergo the bleed out of these components and discoloration on the surface thereof with time. Appearance worsening, such as discoloration and blooming due to the bleeding out during storage in shops may also cause lowering their commercial value. For these reasons, weatherability improvement by rubber components themselves has been required.
Although improving weatherability by blending ethylene-propylene-diene rubber (EPDM) to styrene-butadiene rubber has been studied in order to solve these problems, another problem occurs that styrene-butadiene rubber and EPDM tend to cause phase separation during heat crosslinking, resulting in insufficient fatigue-resistance.
The present applicants have proposed a rubber composition containing not only a random copolymer rubber consisting of structural units derived from ethylene, an α-olefin, and a specific triene compound, but also a diene rubber, carbon black, and a vulcanizer (See Patent document 4). This rubber composition is suitable for a tire sidewall use since the ethylene/α-olefin/triene random copolymer rubber shows fast vulcanization rate comparative to that of the diene rubber and consequently it is unlikely to cause phase separation from the diene rubber with no damage of the excellent mechanical strength intrinsically possessed by diene rubbers.
The present applicants have also found and proposed a rubber composition obtained by mixing a composition containing a non-conjugate polyene copolymer containing structural units derived from an α-olefin and from a non-conjugate polyene and a softening agent, and a diene rubber, and the rubber composition is suitable for forming a tire excellent in braking performance and fuel economy performance (See Patent documents 5 and 6).
Recently in tire production, there is mainly adopted a process including steps of forming an uncrosslinked composition mainly composed of a diene rubber such as styrene/butadiene rubber and natural rubber, for example, into a sheet-like shape, subsequently crosslinking only the surface of the shaped composition by electron beam to prevent sagging, and then assembling into a tire shape, followed by sulphur-crosslinking.
Further, diene rubbers such as natural rubber (NR), styrene-butadiene rubber (SBR), and butadiene rubber (BR) are known as a rubber excellent in dynamic fatigue resistance and dynamical properties, and are used as a raw material rubber for car tires and for anti-vibration rubbers. However nowadays, environmental conditions under which these rubber products are used have been greatly changed, and improvement in thermal resistance and weatherability of the rubber products has been required. For car tires, tread and tire sidewall are particularly required to have weatherability. However, there is so far no rubber having not only excellent mechanical properties, fatigue-resistance, and dynamical property, all being possessed by current diene rubbers, but also good weatherability.
For this reason, there are studied various blended rubber compositions of a diene rubber and an ethylene/α-olefin of 3 to 20 carbons/non-conjugate polyene copolymer such as ethylene/propylene/non-conjugate diene copolymer rubber (EPDM), the diene rubber being excellent in mechanical properties, dynamic fatigue resistance, and dynamical properties, and the copolymer being excellent in thermal resistance and weatherability. However, because the dynamical properties of the ethylene/α-olefin of 3 to 20 carbons/non-conjugate polyene copolymer and those of the diene rubber are at different levels, no blend-based rubber composition exhibiting uniform physical properties has been achieved. Note that the dynamical properties in car tires are used only to estimate whether a subject material worsens fuel economy or not, and the index of the properties is tan δ (loss tangent), a lower value of which means better dynamical properties.
On the other hand, anti-vibration rubber products for cars based on natural rubber which is a currently used diene rubber have been made difficult to provide a fatigue resistance sufficient to practical use due to increase in temperature in engine rooms. Accordingly, a novel rubber material is required which is excellent in thermal resistance, and exhibits mechanical properties, dynamical properties, and fatigue-resistance which are equal to or better than those of diene rubbers.
In general, in order to improve the dynamical properties, crosslinking density must be high. However in existing techniques, in order to make the dynamical properties of ethylene/α-olefin of 3 to 20 carbons/non-conjugated polyene copolymers be comparative to those of diene rubbers such as NR, the crosslinking density becomes too high, resulting in degradation of mechanical properties such as tensile break elongation and causing incompatibility between the dynamical properties and the physical properties.
In this situation, the applicants propose; an anti-vibration rubber composition containing a specific ethylene/α-olefin/polyene amorphous copolymer, a crosslinking agent, and a filler, which composition is able to reduce the used amount of the organic peroxide crosslinking agent and has an excellent thermal resistance, a durability equal to or larger than that of natural rubber, and also exhibits an excellent dynamic-to-static modulus ratio (See Patent document 7); a rubber composition containing an ethylene/α-olefin/non-conjugated polyene copolymer, a powdered silica, a metal unsaturated carboxylate, and an organic peroxide, which composition not only is excellent in dynamical properties and mechanical strength, but also has an improved heat aging resistance (See Patent document 8); and an anti-vibration rubber composition containing a specific ethylene/α-olefin/non-conjugated polyene copolymer having a small B value and containing a powdered silica, a metal unsaturated carboxylate, and an organic peroxide, which composition is not only excellent in thermal resistance and fatigue resistance, but also well-balanced between dynamical properties and mechanical strength (See Patent document 9).
In anti-vibration rubbers made from ethylene/α-olefin/non-conjugated polyene copolymers, in order to improve anti-vibration property, in other words, decrease dynamic-to-static modulus ratio, it has been thought to be effective to use copolymers with a high molecular weight, to suppress the used content of fillers, and to increase crosslinking density, and various processes to achieve these strategies have been studied.
However, there is a problem that ethylene/α-olefin/non-conjugated polyene copolymers with a high molecular weight are difficult to knead because of the high viscosity of the polymers themselves. Moreover, there is also a problem that the increase in crosslinking density, required to become higher as mentioned above in order to improve anti-vibration property, in some cases causes deterioration of mechanical physical properties such as elongation property. In addition, anti-vibration rubber products such as anti-vibration rubbers for an automobile use and specifically for muffler hanger are required to have a particularly high thermal resistance.